Heavy metals such as cadmium, lead and mercury are non-essential elements for plants and are even hazardous to the growth of plants. They are deemed as heavy-metal pollutants for plant growth and food safety. The excessive accumulation of these hazardous heavy metals in food will enable them to enter into the food chain, and even threaten human health. According to the results of a quality and safety survey of rice in various regions of China in 2002 and 2003 made by the Quality Inspection and Supervision Center of Rice and Rice Products, Ministry of Agriculture, one of the quality and safety issues of rice is the over-the-limit content of heavy metals such as cadmium and lead. The over-the-limit rate is more than 10%. Three industrial wastes, non-ferrous metal mining and sewage irrigation are causes that may lead to an excessive amount of toxic heavy metals in the soil and excessive absorption by the plant, being the main source of the accumulation of heavy metals in plants or crops. Therefore, strict limiting criteria for heavy metals in soil and foods (or grains) have been established in various countries. For example, in China, the limit of cadmium in cereals is 0.2 mg/kg, that of lead 0.2 mg/kg and that of mercury 0.02 mg/kg.
Cadmium, lead and mercury that are accumulated in plant foods, such as rice, barley and wheat, are mainly absorbed by the roots from the soil and finally accumulated in the harvest parts after flowing to the top upon transpiration. Research shows that the contents of cadmium, lead and mercury in the soil, especially the effective content (namely absorption by the roots) are the key factors when the roots absorb cadmium, lead and mercury from the soil. Hence, to reduce and control the effective cadmium, lead and mercury content in the soil by various agronomic means is always a key study subject at home and abroad. For instance, lime is applied on acidic and slightly acidic soil to raise the pH value of the soil, which obviously decreases the effectiveness of cadmium, lead and mercury in the soil and finally reduces the absorption of such heavy metals by the roots. However, the application of lime has also a lot of side effects. For example, it may cause the rise of the pH value in the soil, leading to the reduction of the required contents of multiple microelements like iron, manganese and zinc while decreasing such heavy metals, easily resulting in nutrient deficiency in the crops and their failure to thrive. On the other hand, the considerable difference of pH values and pH buffer capacities for different regions and soils gives rise to a great difficulty in accurately controlling the lime application amount. As to paddy rice, the inundation can facilitate the increase of soil reducibility, and the increase of elements such as ferrous iron in the soil and the promotion of the deposit of cadmium, lead and mercury in the form of sulfide, on one hand, decreases the soil effectiveness of these heavy metals. On the other hand, a decrease in the roots' absorption capacity for these elements, greatly reduces the roots' absorption and accumulation of toxic heavy metals from the soil. But for paddy rice itself, long-term inundation is not conducive to its growth and is apt to cause an increase of arsenic absorption in rice.
It is a key orientation of studies around the world to add a curing agent or adsorbent in the soil to combine and fix or absorb heavy metals in the soil like cadmium, lead and mercury. The curing agent or adsorbent reported mainly includes zeolite, kieselguhr, sepiolite, bentonite and limestone, and even alkaline cinder, but there is still no curing agent or adsorbent for heavy metals in the soil that is produced and applied broadly. The mainly reasons lie in their being non-environmentally-friendly, high cost and no popularization efforts as to their use value.